Backport PR #51568 on branch 2.0.x (ENH: Series.fillna with empty argument making deep copy) (#51580)

Backport PR #51568: ENH: Series.fillna with empty argument making deep copy

Co-authored-by: Patrick Hoefler <[email protected]>

Author: meeseeksmachine

pandas/core/generic.py

@@ -6850,7 +6850,7 @@ def fillna(
                         # test_fillna_nonscalar
                         if inplace:
                             return None
-                        return self.copy()
+                        return self.copy(deep=None)
                     from pandas import Series
 
                     value = Series(value)

pandas/tests/copy_view/test_interp_fillna.py

@@ -232,3 +232,27 @@ def test_fillna_interval_inplace_reference(using_copy_on_write):
         assert np.shares_memory(
             get_array(ser, "a").left.values, get_array(view, "a").left.values
         )
+
+
+def test_fillna_series_empty_arg(using_copy_on_write):
+    ser = Series([1, np.nan, 2])
+    ser_orig = ser.copy()
+    result = ser.fillna({})
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(ser), get_array(result))
+    else:
+        assert not np.shares_memory(get_array(ser), get_array(result))
+
+    ser.iloc[0] = 100.5
+    tm.assert_series_equal(ser_orig, result)
+
+
+def test_fillna_series_empty_arg_inplace(using_copy_on_write):
+    ser = Series([1, np.nan, 2])
+    arr = get_array(ser)
+    ser.fillna({}, inplace=True)
+
+    assert np.shares_memory(get_array(ser), arr)
+    if using_copy_on_write:
+        assert ser._mgr._has_no_reference(0)